A granulator is used to reduce the size of plastic or other materials to particles small enough to be used in reprocessing, disposal, etc. This size reduction is accomplished through the use of knives to cut the material into smaller pieces. A granulator has one or more stationary (bed) knives and two or more rotating (rotor) knives. When a rotor knife passes a stationary knife, a cutting action is produced, assuming that a piece of material is located between the knives. A half cylinder containing a large number of perforations (screen) is located directly below the knives to control the size of granulated particles leaving the cutting area. Particles are forced to remain in the cutting area until they are small enough to fall through the screen.
Granulators have been manufactured for over thirty years. The traditional approach to design has been to make the knife large enough to accommodate several round holes for the purpose of bolting the knife to its mating structures. Knives are currently manufactured from a solid piece of tool steel, which is usually "hardened" using a thermal heat treatment process. Once knives become dull, they are typically removed from the machine, resharpened, and installed back into the granulator. A typical knife can be resharpened about five times before it becomes too small to be effective and must be disposed of. When a knife is disposed of, it still contains approximately ninety percent of its original material.
A problem with conventional knives is that as they become dull, the quality of the granulated material suffers. Dulled knives tend to "beat" or "hammer" the material rather than cut it. As a result, the amount of dust or "fines" in the granulate increases. Users wish to reduce the amount of fines to a minimum, because the dust is objectionable from a housekeeping standpoint. It causes problems in material transfer systems, and usually results in waste.
If knives could remain sharp for a longer period of time, the costs associated with resharpening and changing the knives would be reduced. The quality of the granulated material would also be improved.
Extending the life of the cutting edge requires a more durable material; i.e., a material that wears better and is not brittle. Such materials do exist. The real problem arises when one attempts to use these exotic materials in a conventional knife configuration. The cost per pound of these materials is significantly higher than conventional tool steels, and this approach is just not economically attractive.
Over the years there have been many attempts to improve the life of granulator knife cutting edges. The development of better tool steels has caused some increases in knife life, but at a higher cost. For instance, some standard granulator knives are manufactured from chrome-vanadium-steel (CVS). Better life can be obtained from a knife made from hardened D-2 tool steel, but the cost is about 40 percent higher. Still greater life can be achieved by treating a knife surface with a flame sprayed tungsten or titanium carbide coating. Knife life can triple, compared to that of a CVS knife, but not without a substantial increase in cost.
Other coatings on conventional knives have been tried with the goal of improving edge life. Titanium nitride coatings, which have dramatically improved the life of such things as drill bits, have been tried. Although some improvement was achieved, the economics are not attractive.
Strips of tungsten carbide have been secured to mild steel knife bodies. The tungsten carbide is very hard and would enhance the wear characteristics, while the majority of the knife is made from lower cost mild steel. Although this approach has been tried, the difficulties in joining the two metals made this a very expensive process, and therefore not a good solution to the knife wear problem. In a similar fashion, D-2 tool steel strips were attached to mild steel bodies. The approach also did not prove to be reliable or economical.
A more modern approach is to metallurgically bond (using the hot isostatic press approach (HIP)) a small amount of a hardenable ceramic composite to a conventional tool steel body in an attempt to achieve an economical solution to knife wear. The ceramic composites typically consist of tungsten or titanium carbide particles suspended in a tool steel matrix. The product is machinable before heat treatment, after which it requires grinding. The cost of this process appears to be very high and its economics are questionable.
Another approach tried was to apply a cladding to the knife tip area on a mild steel knife base. After the very hard weld deposited material was applied, the final knife cutting edge had to be machined through an electrical discharge machining (EDM) process. The economics of this approach did not prove to be acceptable.
A small reversible, and expendable knife Turnknife.TM. is available. Its complex shape is produced by extrusion from a high durability proprietary steel, and the final critical features are produced by grinding. This knife is positioned and retained to its mounting surface by a clamp. The knife requires complex and precise features on both its mounting surface and its clamp. In addition, the knife cannot cut thick parts because of its small protrusion from the rotor.
My U.S. Pat. No. 5,097,790 teaches a reversible, trapezoidal shaped knife which is miniature in size and is held in place by a bolted clamp. This knife has many advantages over conventional knives but is limited to cutting parts which have light to medium cross-sections. Being reversible and precisely located on the rotor by two pins, the protrusion of this knife from the front edge of the rotor has a practical limit. Because this amount of protrusion affects its ability to cut through the material, this knife is not a good choice for cutting parts with very thick cross-sections, such as from 3/8" to 11/2" for many materials.
To solve this problem, as disclosed in the parent application, a clamp-knife design was developed which used only `exotic material` and was capable of cutting parts as thick as with conventional knives, such as up to 11/2" for many materials and shapes. The knife used only about 12% of the materials of conventional knives.
The knife was designed to be resharpened at least three times. The rear face of the knife was butted up against a ledge, which ledge was formed either in the clamp or the rotor seat. This ledge was fixed. When a new knife was placed in the seat and clamped into position, it extended its maximum unsupported distance from the clamp and seat. This clamp-knife design was not designed for use in abusive conditions. That is, if the granulator were designed for granulating plastic material, such as scrap plastic from a molding machine and used in-house, the knife would only expect to cut plastic. However, it was found in actual use even where a granulator was used in-house where only scrap plastic should be encountered, metal parts such as bolts, would find their way into the granulator and the knife, because of its extension from the clamp and seat, would tend to break.
Broadly, the present invention embodies a rotor seat-knife-adjustable clamp design which overcomes the breaking of the knife in abusive conditions and which allows the knife to be resharpened and precisely repositioned on the average at least three times. Further, the knife is smaller than conventional knives and uses only about 12% of the material used in conventional knives for the same purpose.
The knife is trapezoidal in cross-section and includes a rear wall, an upper surface and a sloped surface which terminates in a cutting edge. Where the upper surface and sloped surface intersect they define a knife alignment edge.
The rotor seat has a floor and rear wall formed therein. The knife is seated on the floor with its rear edge butted up against the rear wall. The rotor seat is also characterized by tapped holes.
The clamp includes a plurality of holes. The wall defining each hole is characterized by a stepped surface which defines a first upper hole of a diameter greater than the bolt head which will pass therethrough and a second narrower lower hole of a diameter greater than the shaft of the bolt which passes therethrough. The clamp is further characterized by a rear wall and a forward portion, the forward portion having a sloped upper surface, and a flat bottom surface which intersect at a clamp alignment edge.
The enlarged holes in the clamp allow the clamp position to be adjusted each time a knife is seated. When a knife is seated it engages the rear wall of the rotor seat. The location of the rear wall is constant or fixed. A gauge is used to position the clamp with reference to the knife edge, which is immovable, and the rear wall of the clamp, which is moveable. The gauge, which is a single U-shaped piece, locates the clamp with reference to the knife such that the sloped surface of the knife and the sloped upper surface of the adjustable clamp lie in substantially the same plane and the knife alignment edge and clamp alignment edge are adjacent to one another. This ensures that the knife receives maximum support from the clamp.
Once a cutting edge becomes dull,, the knife is removed and resharpened. The resharpened knife is then placed on the floor of the rotor seat with the rear wall of the knife butted up to the rear wall of the rotor seat. Dimensionally the knife is smaller with reference to the distance between its rear wall and the cutting edge. Again, the gauge is used, typically one gauge at each end of the knife, and the clamp is moved until it is properly positioned such that the knife alignment edge and clamp alignment edge are adjacent to one another. That is, the clamp is now properly aligned with reference to the new size of the knife to provide maximum support to the knife. It is important to advance the clamp sufficiently toward the edge of the knife to provide maximum support, but not too far forward such that it would interfere with (hit) the stationary (bed) knife. Once the knife is properly positioned then the bolts are secured.
The knife and clamping arrangements described herein dramatically improve the economics of attaining longer lasting knife edges, reduce machine downtime and labor costs due to less frequent changes, an improves granulate quality by minimizing fines.